Amyotrophic lateral sclerosis (ALS) is the most common, devastating, adult-onset neuromuscular degenerative disease. The underlying pathology is characterized by selective loss of motor neurons in spinal cord, brainstem, and cerebral cortex. ALS usually progresses rapidly and leads to total paralysis concomitant with respiratory failure within 2 to 5 years after diagnosis. Although ALS is generally considered to be a neuromuscular disorder, it is now recognized as a multi-system neurological disease. The annual incidence of ALS is 1 to 2 cases per 100,000 persons. Approximately 10% of ALS cases are familial ALS (FALS) cases that are inherited genetically; the remaining cases are sporadic ALS (SALS) cases whose cause(s) is less understood.
Although a cure for ALS does not exist, disease progression can be ameliorated. Survival rates of ALS patients can be improved by early diagnosis and medical intervention including the use of medications such as Riluzole and multi-disciplinary care such as the use of noninvasive positive-pressure ventilation and percutaneous endoscopic gastrostomy. Conventionally a diagnosis of ALS is made by a neurologist primarily on the basis of clinical symptoms and electrophysiological findings since the turn of nineteenth century. A 10% misdiagnosis rate has been reported for ALS. In addition, the period between onsets of symptoms to confirmation of ALS diagnosis may range from 13 to 18 months. Recent studies using proteomic approaches have identified several potential biomarkers for the diagnosis of ALS, but none of these markers is effective in clinical practice.
In FALS, mutations in the gene encoding copper/zinc-dependent superoxide dismutase 1 (SOD1) contribute to the disease process. An estimated 20% of FALS cases are caused by mutations of SOD1. Mutated SOD1 cannot catalyze dismutation of superoxide anion, resulting in oxidative stress and death of motor neurons. Mutations in SOD1 that increase protein function also contribute to the disease progression. Identification of the role of SOD1 in FALS has led to the development of drugs that target SOD1.
Another important molecule involved in FALS is D-amino acid oxidase (DAO), an enzyme that controls levels of the neurotransmitter D-serine, which activates N-methyl-D-aspartic acid receptors. DAO controls levels of D-serine by oxidizing D-amino acids into α-keto acids and ammonia with hydrogen peroxide produced as a side product. In 2010, Belleroche et al. discovered a unique missense mutation in DAO encoding DAOR199W in a family with a strong inheritance of FALS. The same group demonstrated ubiquitin aggregation and cytotoxicity in a DAOR199W-transfected motor neuron cell line. Millecamps et al. also identified a rare mutation in DAO encoding DAOR38H in French patients with FALS.
A third biomarker for FALS, TAR DNA-binding protein 43 (TDP-43), is found in the inclusion bodies of neuronal cells. A mutation in TDP-43 derived from a missense single nucleotide polymorphism (SNP), TDP-43M337V, causes the aggregation of poly-ubiquitins and damages neurons. Furthermore, 13 mutations in the gene encoding the splicing protein fused in sarcoma/translated in liposarcoma (FUS/TLS) are associated with FALS.
Recently, missense mutations of the genes for FIG4, an phosphatidylinositol 3,5-bisphosphate 5-phosphatase, and angiogenin (ANG) were discovered in both FALS and SALS. A mutation in FIG4 causes peripheral neuropathy, and a mutation in ANG is predominantly associated with ALS cases of Celtic origin. SNPs in the genes for several other biomarkers also appear to be associated with ALS.
Because more than 90% of ALS cases are SALS, studies on SALS are important for understanding causative disease mechanisms as well as developing new diagnostic tools and treatment strategies. In 2010, transthyretin was identified as an important biomarker for post-translational modification. Transthyretin levels are lower in patients with ALS, particularly in patients with SALS, than those of healthy controls; however, post-translational modification of transthyretin was higher in patients with ALS than those of healthy controls. These results indicate that non-genetic factors may also be involved in the development of SALS.
High mobility group box 1 protein (HMGB1) is a non-histone chromosomal protein. As a DNA binding protein, HMGB1 is involved in the maintenance of nucleosome structure and the regulation of gene transcription. It is also active in DNA recombination and repair. HMGB1 is a late mediator of endotoxemia and sepsis. It is released from activated macrophages, induces the release of other proinflammatory mediators, and mediates cell death when overexpressed. In the spinal cord of a SOD1_G93A transgenic mouse of ALS, degenerating neurons showed a reduction of HMGB1 immunoreactivity, suggesting an extracellular release of HMBG1. In contrast, in reactive glial cells HMGB1 was remarkably expressed in the nucleus, but not in the cytosol, likely contributing to proliferation and/or hypertrophy of these cells (Bendotti et al., Neurosci Lett. 2007, 412 (1):73-77). In that study, the expression and cellular distribution of TLR2, TLR4, RAGE and their endogenous ligand HMGB1 in the spinal cord of control (n=6) and SALS (n=12) patients was investigated. In ALS spinal cord, HMGB1 signal is increased in the cytoplasm of reactive glia, indicating a possible release of this molecule from glial cells (Aronica et al., Neuroscience, 2011, 179: 233-243).